Regulated or controlled power supplies are found in virtually all electronic devices, including for example battery chargers, cellular telephones, computers, computer monitors, televisions, audio equipment, video cameras, navigation devices or portable media players. A typical power supply is a converter, such as a direct current to direct current converter (in the following simply designated as DC to DC converter). The DC to DC converter accepts a DC input voltage and produces a DC output voltage. Typically, the (mean) output voltage produced is at a higher or lower value than the input voltage according to whether a up converter, sometimes called a boost or step-up converter, or a down converter, sometimes called a buck or step-down converter, is respectively used.
Using a technique called “synchronous rectification”, DC to DC converters are preferably implemented as synchronous DC to DC converters in a switched operating mode, while replacing the rectifying means such as the common flywheel diode usually connected in series with inductive energy storage means, such as external coils or inductors, by switching means, such as transistors. Thus, such a synchronous DC to DC converter usually comprises at least inductive energy storage means and switching means, the switching sequence of which is controlled by control means for providing electrical energy from the inductive energy storage means to the output of the synchronous DC to DC converter.
Most of these control means currently operate either at fixed frequency in Pulse Width Modulation (PWM) or continuous mode, when the inductor current is continuous while never reaching a zero value, or at variable frequency in Pulse Frequency Mode (PFM) or discontinuous mode, when the inductor current is discontinuous while pulsing from a zero value to a positive value.
The control means operating in PWM mode are rather suitable for relatively high output load, whereas those operating in PFM mode are rather suitable for relatively low output load. They also present the advantage to provide better performance than those operating in PFM mode, but the disadvantage to require a more complex circuit structure.
In PFM mode, the control means for up converters are controlled in peak current, which may disadvantageously be very high, and are required to have a fixed on-time.
There are also hysteretic controllers, but these are rather suitable for down converters, since a hysteretic up converter would require a means of sensing the inductor current that would cost power and be difficult to realize at high frequencies due to ringing and spikes on the inductor voltage.